Various systems have been developed to provide position, velocity, and/or attitude information to a user. Inertial based navigation systems (INS) that track changes in position and attitude from an initial point are capable of providing position, velocity, and/or attitude information that is accurate and low in noise. However, depending on the quality of sensor used, INS may suffer from excessive position drift over time. In addition, initial point information may not be readily available at all times, so determination of a navigation solution relative to an absolute frame of reference (such as the earth) may not be possible. Radio based systems, including satellite navigation systems such as global navigation satellite systems (GNSS) (for example, global positioning system (GPS)), are capable of providing low drift information, but the data tends to be noisy and subject to radio propagation error such as multipath. In certain environments, such as urban canyons, radio propagation paths may be completely blocked, rendering satellite navigation system information completely unavailable for extended periods of time.
For high performance applications, INS and satellite navigation systems may be combined into a single system to produce a navigator that demonstrates a low noise and low drift navigation solution relative to an earth reference frame. However, urban environments usually have situations that limit the availability of the satellite navigation system information (e.g. urban canyons), rendering an INS/satellite navigation system inadequate for high performance applications. One solution to the blockage problem is to erect a local radio network whose elements have access to absolute position data. The network can be used to supply position information to a user via triangulation. However, a local radio network capable of providing a 3D solution requires three radio transmitters, requiring a significant infrastructure and access to absolute position at three separate points.